SECOND problem: why qt-creator installs with prefix=/ but not prefix=/usr/ ?

1)Apparently the upstream did some changes and an make the patch invalid. I ll modify it . Thanks for mention it. As you understand its hard to support live ebuilds cause the upstream is doing changes all the time

2)qt creator doesnt install much in your system. Just 5-6 libs and a binary file .

As i said before, dont show us only the error. I need to see the whole build.log file to understand what is going on. But since it compiles fine here I would say to try again. Maybe upstream was messing around but now its fixed